Currently, surfactants for detergent are required to exhibit not only a high detergency but also high environmental adaptability and safety and low irritation property. From this point of view, practical utilization of fatty acid amide surfactants, having excellent biodegradability and safety, has been investigated. Among these investigations, a practical utilization of polyoxypropylene fatty acid isopropanolamide surfactant as a specific type of the fatty acid amide surfactants having very low irritation property has been studied.
Japanese Unexamined Patent Publication No. 9-20740 discloses polyoxypropylene fatty acid isopropanolamide surfactants. The polyoxypropylene fatty acid isopropanolamide surfactants have, in the molecular structure thereof, a secondary amide bond (—CONH—) which is chemically stable, and thus exhibit excellent stability over a wide range of pH value. Also, the polyoxypropylene fatty acid isopropanolamide surfactants exhibit excellent thickening performance, foam-enhancing performance, foam-stabilizing performance, emulsifying performance, dispersing performance and solubilizing performance. However, the surfactants are disadvantageous in color stability and the smell thereof. Thus, for the practical utilization of the surfactants, these disadvantages must be removed. Further, the polyoxypropylene fatty acid alkanolamide surfactants produced in accordance with the production process disclosed in the Japanese publication may exhibit a sensitivity under same conditions of the production process, and thus it is strongly desired that the cause of the sensitivity is investigated and the process is improved.